Spectrum allocation is the problem of assigning carrier frequencies to NOMA groups. Frequency-based OMA techniques such as orthogonal frequency-division multiple access (OFDMA) can still be used to distinguish NOMA groups within a beam. However, due to THz-band high frequency-selectivity, hardware constraints and molecular absorption, THz path losses are more severe at particular sub-bands that need to be avoided. The available bandwidth is thus divided into smaller transmission windows, the centers of which enjoy less path loss than the sides [1]}. For instance, a total bandwidth of \([45]{GHz}\)  is available for transmission at a central frequency of \([300]{GHz}\)  (transmission window from \(227.5\)  to \([322.5]{GHz}\) ) [1]}. Efficient THz carrier frequency assignment in OMA can leverage this property to allocate window centers to faraway users and window sides to closer users in a distance-aware multi-carrier (DAMC) approach [1]}. The same concept can be applied to NOMA by allocating each NOMA group a sub-band at the transmission window center or side, depending on the distance from the BS. To this end, a group distance metric needs to be defined, the center of mass of user distances in a group, for example. The distinction between the group distances is more clear if users are clustered using the SDG scheme described in Sec. REF .
